Bump supertest from 6.1.3 to 6.3.0 in /rendertron

Bumps [supertest](https://github.com/visionmedia/supertest) from 6.1.3 to 6.3.0.
- [Release notes](https://github.com/visionmedia/supertest/releases)
- [Commits](ladjs/supertest@v6.1.3...v6.3.0)

---
updated-dependencies:
- dependency-name: supertest
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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